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Unformatted text preview: Advanced information on the Nobel Prize in Physics, 7 October 2003 Information Department, P.O. Box 50005, SE-104 05 Stockholm, Sweden Phone: +46 8 673 95 00, Fax: +46 8 15 56 70, E-mail: firstname.lastname@example.org, Website: www.kva.se Superfluids and superconductors: quantum mechanics on a macroscopic scale Superfluidity or superconductivity which is the preferred term if the fluid is made up of charged particles like electrons is a fascinating phenomenon that allows us to observe a variety of quantum mechanical effects on the macroscopic scale. Besides being of tremendous interest in themselves and vehicles for developing key concepts and methods in theoretical physics, superfluids have found important applications in modern society. For instance, superconducting magnets are able to create strong enough magnetic fields for the magnetic resonance imaging technique (MRI) to be used for diagnostic purposes in medicine, for illuminating the structure of complicated molecules by nuclear magnetic resonance (NMR), and for confining plasmas in the context of fusion-reactor research. Superconducting magnets are also used for bending the paths of charged particles moving at speeds close to the speed of light into closed orbits in particle accelerators like the Large Hadron Collider (LHC) under construction at CERN. Discovery of three model superfluids Two experimental discoveries of superfluids were made early on. The first was made in 1911 by Heike Kamerlingh Onnes (Nobel Prize in 1913), who discovered that the electrical resistance of mercury completely disappeared at liquid helium temperatures. He coined the name superconductivity for this phenomenon. The second discovery that of superfluid 4 He was made in 1938 by Pyotr Kapitsa and independently by J.F. Allen and A.D. Misener (Kapitsa received the 1978 Nobel Prize for his inventions and discoveries in low temperature physics). It is believed that the superfluid transition in 4 He is a manifestation of Bose-Einstein condensation, i.e. the tendency of particles like 4 He that obey Bose-Einstein statistics to condense into the lowest-energy singleparticle state at low temperatures (the strong interaction between the helium atoms blurs the picture somewhat). Electrons, however, obey Fermi-Dirac statistics and are prevented by the Pauli principle from having more than one particle in each state. This is why it took almost fifty years to discover the mechanism responsible for superconductivity. The key was provided by John Bardeen, Leon Cooper and Robert Schrieffer, whose 1957 BCS theory showed that pairs of electrons with opposite momentum and spin projection form Cooper pairs. For this work they received the 1972 Nobel Prize in Physics. In their theory the Cooper pairs are - 1 - structureless objects, i.e. the two partners form a spin-singlet in a relative s-wave orbital state, and can to a good approximation be thought of as composite bosons that undergo Bose-Einstein condensation into a condensate characterised by...
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